The present invention relates generally to the production of laminated paper products and more particularly to a method of lamination and a device for laminating a continuous roll of plastic film material to a continuous roll of paper backing material wherein the device has a system for controlling the tension in the paper material to ensure proper lamination of the paper material with the film material.
Lamination of a plastic film material to a paper material has been found to be particularly advantageous where paper products having high strength and abrasion resistant qualities are required. Such laminated composites are particularly desirable in the packaging industry, where artwork may be printed on the interior side of the plastic film to produce a high quality display package. The construction and use of such laminated composites are described in Peer, U.S. Pat. No. 4,254,173 and in Haake et al., U.S. Pat. No. 4,496,417 and U.S. patent application Ser. No. 624,480, filed June 22, 1984 by James Wendell Jensen, et al. (now U.S. Pat. No. 4,572,752) which are hereby incorporated by reference for all that is disclosed therein.
In producing a laminated composite of the type described in Peer, both the plastic film material and the paper material may be provided in continuous sheets from spools. The paper and the plastic film generally pass over a number of roller type devices where each of the materials are independently stretched out under tension and treated as necessary depending upon the particular end use for the laminated composite. For example, the plastic material may be irradiated in conventional processes to increase its strength or improve its printability. In preferred embodiments, the plastic is printed with various graphics and provided with a metalized background over the graphics to enhance the package appearance. The paper may undergo treatment as well such as being electrostatically charged to aid in the bonding process. Either the film material or the paper material or both are treated thereafter with suitable adhesive to provide a bond between the paper and film. To complete the laminating process, the paper and film material are pressed together between opposed rollers to produce a smooth flat composite. Various heating or cooling processes may also be required to ensure proper adhesion of the surfaces, depending on the type of the film, paper and adhesive agent which are being used in the process. The end product of the process is a laminated composite which may be fed directly to cutting dies or other machines for further processing. The composite may also be taken up directly on a separate spool for storage and later processing.
Use of the film/paper composite provides many advantages over conventional packaging material but also creates some unique problems. In order to be cost effective, the plastic material used is generally quite thin; on the order of 0.25-2.0 mils. Plastic films of such thicknesses tend to stretch or shrink during the lamination process as a result of variations in temperature and the different inertial forces exerted on the film as it is unrolled and processed prior to bonding with the paper. Such stretching and shrinking may cause warping or buckling in the laminated composite as the film returns to a steady state condition. Similarly, a printed image may be repeated on the film for later lamination, in registry with predetermined lengths of paper material. In such cases, the printed image length must be held within close tolerances in order to permit proper registry with other processing activities such as automated cutting in a later process step.
Apparatus for controlling the distortion of an extensible plastic film web used in a film/paper laminate are disclosed in the above referenced patent of Haake et al. and patent application of Jensen et al. It has been discovered by applicants that even minor tension variations in the paper web or the composite in such laminating apparatus may influence the quality of laminate that is produced. Such paper web and composite web tension variations may be due to a number of variables such as speed variations in the paper unwind spool or composite takeup spool due to changing spool size and/or spool eccentricity; variations in the speed of the laminating rolls due to tension changes in the film web associated with film web stretch control; variations in the speed of the laminating roll due to speed fluctuations in the paper unwind roller laminate takeup roll; variations in the speed of the laminating rolls due to drive motor voltage and current fluctuations or any combination of the above or other variables.
Applicants have discovered that by providing a plurality of isolated tension zones in the paper web and the composite web and by maintaining the tension in the different zones at relatively constant values through the use of electronic control systems, an improved quality laminate may be obtained. Applicants have further discovered that in a paper web having two such isolated tension zones upstream of the laminating nip that an improved laminate may be achieved by maintaining the tension in the relatively upstream zone above the tension in the relatively downstream zone.